In the 3GPP (3rd Generation Partnership Project) which is a standardization project, through adoption of an OFDM (orthogonal frequency-devision multiplexing) communication scheme and flexible scheduling which is called a resource block and which is based on a predetermined frequency and time unit, Evolved Universal Terrestrial Radio Access (hereinafter, referred to as “EUTRA”) in which high-speed communication is achieved has been studied, and Advanced EUTRA (also referred to as LTE-Advanced) which is a developed form of EUTRA is being studied.
In Advanced EUTRA, carrier aggregation has been proposed as a technique which enables higher-speed data transfer while maintaining compatibility with EUTRA. Carrier aggregation is a technique in which data which is transmitted from a transmitting device by using different frequency bands (also referred to as “carrier frequencies” or “component carriers”) is received by a receiving device supporting the different frequency bands so that the data transmission rate is increased. Hereinafter, a receiving device in downlink transmission is referred to as a “mobile station”, and a transmitting device is referred to as a “base station”. A receiving device in uplink transmission is referred to as a “base station”, and a transmitting device in uplink transmission is referred to as a “mobile station”. The scope of application of the present invention is not necessarily limited to these devices. For example, a receiving device in downlink transmission and a transmitting device in uplink transmission may be a relay station.
In Advanced EUTRA, one downlink component carrier and one uplink component carrier are combined so as to form one cell. Instead, a cell may be formed of only one downlink component carrier. A cell is allocated to (configured for) a mobile station through signaling in the RRC layer by a base station. A cell allocated to a mobile station has two states, the deactivated state in which radio transmission is not allowed to be performed and the activated state in which radio transmission is allowed to be performed. A mobile station uses one cell called a primary cell and zero or more cells called secondary cells to communicate with a base station, and the primary cell is always in the activated state. All of the secondary cells allocated to a mobile station are switched to the deactivated state upon occurrence of a handover, and signaling (Activation command) which is performed by a base station after the handover and which represents an explicit instruction for activation causes the secondary cells to be switched to the activated state.
In the case where an initial connection to a base station is to be established, where a handover is to be performed, or where uplink data transmission or downlink data reception needs to be performed in a state in which the uplink is not synchronized, a mobile station in EUTRA performs a process for uplink transmission timing adjustment (uplink synchronization establishment) which is called a random access procedure. In Advanced EUTRA, when multiple cells are allocated to a mobile station, a random access procedure in the primary cell causes the uplink component carriers for all of the cells which are in the activated state to be adjusted to the same transmission timing. There is a timer (referred to as a “transmission timing timer”) which is restarted every time an uplink transmission timing is adjusted (a signal for adjusting transmission timing, which is called a TA command, is received from a base station). When the transmission timing timer has counted a predetermined time without being restarted (when the transmission timing timer is completed) or when the transmission timing timer stops due to another reason, the uplink is regarded as being out of synchronization, and the mobile station cannot perform radio transmission other than a random access procedure (see NPL 1).
A third-generation base station defined by the 3GPP is referred to as a “NodeB”, and a base station in EUTRA and Advanced EUTRA is referred to as an “eNodeB”. A base station manages a cell which is an area in which a mobile station can communicate. A cell is also referred to as a “femtocell”, a “picocell, or a “nanocell” depending on the size of the area in which a mobile station can communicate with a base station. When a mobile station can communicate with a certain base station, a cell of the certain base station is a cell in which the mobile station stays, and a cell of another base station or having a different frequency is referred to as a “surrounding cell”.